CVE-2026-74657: ipv4: Fix fib_nlmsg_size() for RTA_VIA nexthops

From: Greg Kroah-Hartman <[email protected]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

ipv4: Fix fib_nlmsg_size() for RTA_VIA nexthops

fib_nlmsg_size() still estimates nexthop space as if every gateway is
encoded as an IPv4 RTA_GATEWAY attribute. IPv4 routes can also carry an
IPv6 gateway, which fib_nexthop_info() dumps as RTA_VIA.

As a result, route notifications can allocate an skb that is too small.
fib_dump_info() then fails with -EMSGSIZE and rtmsg_fib() hits the
WARN_ON() that marks such failures as a fib_nlmsg_size() bug. With
panic_on_warn set, this becomes a kernel panic.

Mirror the actual nexthop dump layout in fib_nlmsg_size(): account for
IPv6 nexthop gateways dumped as RTA_VIA, for the no-header rtnexthop
layout used inside RTA_MULTIPATH, and for RTA_FLOW only when it is
actually present.

The Linux kernel CVE team has assigned CVE-2026-74657 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.2 with commit d15662682db232da77136cd348f4c9df312ca6f9 and fixed in 5.15.216 with commit 0f0ca602941d0a81ae9514943ca06c55159c6385
	Issue introduced in 5.2 with commit d15662682db232da77136cd348f4c9df312ca6f9 and fixed in 6.1.183 with commit 5307a53599fa762c06e475ee4a375252074fd324
	Issue introduced in 5.2 with commit d15662682db232da77136cd348f4c9df312ca6f9 and fixed in 6.6.152 with commit 7f80ad373ce4a7af5367ff273cea0f16e91387f3
	Issue introduced in 5.2 with commit d15662682db232da77136cd348f4c9df312ca6f9 and fixed in 6.12.104 with commit 4a5dfbae5179f6574695012a980476254df2d295
	Issue introduced in 5.2 with commit d15662682db232da77136cd348f4c9df312ca6f9 and fixed in 6.18.45 with commit a59edda6eda1252340354322d8ab318b2e9052fb
	Issue introduced in 5.2 with commit d15662682db232da77136cd348f4c9df312ca6f9 and fixed in 7.1.9 with commit 9b22f13524fa0de0d963bbd3002df6c28bae3395
	Issue introduced in 5.2 with commit d15662682db232da77136cd348f4c9df312ca6f9 and fixed in 7.2 with commit 4ff9548d84945d2cbf9e4c207288063a200ea397

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74657
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/ipv4/fib_semantics.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/0f0ca602941d0a81ae9514943ca06c55159c6385
	https://git.kernel.org/stable/c/5307a53599fa762c06e475ee4a375252074fd324
	https://git.kernel.org/stable/c/7f80ad373ce4a7af5367ff273cea0f16e91387f3
	https://git.kernel.org/stable/c/4a5dfbae5179f6574695012a980476254df2d295
	https://git.kernel.org/stable/c/a59edda6eda1252340354322d8ab318b2e9052fb
	https://git.kernel.org/stable/c/9b22f13524fa0de0d963bbd3002df6c28bae3395
	https://git.kernel.org/stable/c/4ff9548d84945d2cbf9e4c207288063a200ea397